CARGO TANK VENTING ARRANGEMENTS
1. The cargo tank venting arrangements shall -
.1 be so designed and constructed as to ensure that the pressure, above or below that of the atmosphere, within the tanks does not exceed the design pressures;
.2 include pressure-vacuum valves capable of providing for the flow of vapour, air or inert gas mixtures caused by thermal variations within the cargo tank; and
.3 be capable of providing for the flow of vapour, air or inert gas mixtures whilst the tank is being loaded, ballasted or discharged at the highest rate.
2. Every vent system outlet to atmosphere from a valve required by paragraph 1.1 shall be located as high and at the furthest distance from a source of ignition as is practicable and in no case shall it be located less than 2 metres above the cargo tank deck or less than 5 metres from air intakes or openings to enclosed spaces containing a source of ignition or from machinery and equipment which may constitute an ignition hazard. Anchor windlass and chain locker openings constitute and ignition hazard.
3. By-pass arrangements for the pressure-vacuum valves required by paragraph 1.2 may be fitted if the valves are located in a vent main or masthead riser. Indicators showing whether the by-pass is open or shut shall be provided.
4. Every vent system outlet to atmosphere provided in accordance with paragraph 1.3 shall -
4.1 permit the free flow of vapour mixtures, or be so designed that the discharge velocity of the vapour mixtures is at least 30 metres per second;
4.2 be so arranged that the vapour mixture is discharged vertically upwards;
4.3 where the system permits the free flow of vapour mixtures, be such that the outlets to atmosphere are located at least 6 metres above the cargo tank deck and at least 10 metres measured horizontally, from the nearest air intake or opening to an enclosed space containing a source of ignition and from machinery and equipment which may constitute an ignition hazard; such deck machinery may include anchor windlass and chain locker openings. Any outlet less than 4 metres, measured horizontally, from a fore and aft gangway shall be located at least 6 metres above the gangway;
4.4 where the system is so designed that the discharge velocity of the vapour mixtures is at least 30 metres per second, be such that the outlets to atmosphere are located at least 2 metres above the cargo tank deck and at least 10 metres, measured horizontally, from the nearest air intake or opening to an enclosed space containing a source of ignition and from machinery and equipment which may constitute an ignition hazard. Such outlets shall be provided with high velocity vents designed and constructed in accordance with Schedule 5; and
4.5 be so arranged as to prevent the design pressure of any cargo tank being exceeded. For the purposes of this Schedule the system shall be designed on the basis of the maximum designed cargo loading rate of any tank or group of tanks multiplied by a factor of at least 1.25.
4.6 On ships constructed on or after 1 July 2002, the arrangements for the venting of vapours displaced from the cargo tanks during loading and ballasting shall comply with this schedule and shall consist of either one or more mast risers, or a number of high velocity vents. The inert gas supply main may be used for such venting.
5.1 The venting arrangement of each cargo tank may be independent or combine with other cargo tanks and may be connected to the inert gas piping required by the [Merchant Shipping (Fire Protection: Large Ships constructed before 1 July 2002) Rules 2003] or the [Merchant Shipping (Fire Protection: Large Ships constructed on or after 1 July 2002) Rules 2003] .
5.2 Where the arrangements are combined with other cargo tanks, stop valves or other effective means of isolating each cargo tank shall be provided.
5.3 Stop valves shall be provided with locking arrangements to permit control of their operation. Any cargo tank isolation arrangement provided in accordance with this paragraph shall not prevent the flow of vapour, air or inert gas caused by thermal variations within the tank. In addition, the following requirements shall apply to ships constructed on or after 1 July 2002-
5.3.1 The locking arrangements for the stop valves shall be under the control of the responsible ship's officer.
5.3.2 There shall be a clear visual indication of the operational status of the valves or other acceptable means.
5.3.3 Where tanks have been isolated, it shall be ensured that relevant isolating valves are opened before cargo loading or ballasting or discharging of those tanks is commenced.
5.3.4 Where cargo loading and ballasting or discharging of a cargo tank or cargo tank group which is isolated from a common venting system is intended, that cargo tank or cargo tank group shall be fitted with a means for over-pressure or under-pressure protection as required in paragraph 8.2 of this schedule.
6. The venting system shall be provided with devices to prevent the passage of flame into the cargo tanks. The design, construction, location and testing of these devices shall be in accordance with Schedule 3. The devices for cargo tanks in which the atmosphere is flammable shall be flame arrestors or high velocity vents. The devices for cargo tanks in which the atmosphere is rendered non-flammable by a fixed inert gas system provided in accordance with the [Merchant Shipping (Fire Protection: Large Ships constructed before 1 July 2002) Rules 2003] or the [Merchant Shipping (Fire Protection: Large Ships constructed on or after 1 July 2002) Rules 2003] may be flame screens. In ships constructed on or after 1 July 2002, ullage openings shall not be used for pressure equalisation. They shall be provided with self-closing and tightly sealing covers. Flame arresters and screens are not permitted in these openings.
7. The vents shall be connected to the top of each cargo tank and be self-draining to the cargo tanks. Other permanently installed drainage arrangements may be permitted where it is not possible to provide self-draining vent lines.
8.1 High level alarms or overflow control systems or other equivalent means together with cargo tank content gauges and filling procedures shall be provided to protect the tanks from excess pressure due to overfilling. For ships constructed on or after 1 July 2002, such provisions shall include independent gauging devices and cargo tank filling procedures, but spill valves are not permitted as equivalent to an overflow system.
8.2 A secondary means of allowing full flow relief of vapour, air or inert gas mixtures shall be provided to prevent over-pressure or under-pressure in the event of failure of the arrangements required by paragraph 1 of this schedule. Alternatively, pressure sensors may be fitted in each tank protected by the arrangements required by paragraph 1 with a monitoring system in the ship's cargo control room or the position from which cargo operations are normally carried out. Such monitoring equipment shall also provide an alarm facility which is activated by detection of over-pressure or under-pressure conditions within a tank.
9. Isolation of the slop tanks containing oil or oil residue in combination carriers from other cargo tanks shall be by blank flanges, which shall remain in position at all times when cargoes other than oil cargoes are carried.
10. The master shall be provided with information regarding the maximum permissible loading rate for each cargo tank and, in the case of combined venting systems, for each group of cargo tanks.
